In the last three weeks owners of homes and small businesses along the Allegheny River have taken out more flood insurance policies than were in effect during all of 1975, the year of Hurricane Eloise, a joint government-insurance industry association reports. Eloise alone generated $40 million in property claims against the federally backed National Flood Insurance program.

This time it is the fear of spring thaws and resulting floods that has been driving not only Pennsyvanians but residents of Kentucky, Ohio, West Virginia and even New York to take out policies, says George Bennett, spokesman for the National Flood Insurers Association, the government-industry group based in Arlington. As last week's warming trend occasioned warnings on the part of weather bureau, Pennsylvanians took out 26,700 policies, Bennett said. Together with the two preceding weeks, the number of new policies reached a total of 41,250, he added.

In a recent 30-day forecast, the National Weather Service predicts below-normal seasonal temperatures and below-normal precipitation for the North-eastern quadrant of the country, excepting Maine. Considerable warm weather and/or rain are necessary to break up ice jams and cause flooding, according to the service; gradual melting of snow is not of itself sufficient. Though the long-range forecast does not point to the probability of immediate flooding, officials caution these forecasts have only a 60 per cent accuracy record.

"If the flood does occur," one corporate official told the Wall Street Journal, "it will be the best-planned-for flood since Noah."

Maryland Gov. Marvin Mandel has asked his civil defense chief, Gen. Rinaldo Van Brunt, to coordinate plans with neighboring states. Western Maryland's Garett County received a particularly heavy snow cover this winter - 120 inches.

Residents of flood-prone Oil City, Pa., in Western Pennsylania, have been gearing up for a month, moving valuables and furniture to the second stories of homes, sandbagging doors and windows.

The U.S. Army Corps of Engineers is getting ready for the waters in Amherst, N.Y., a Buffalo suburb of 115,000 residents. The Corps has ordered 50,000 extra sandbags, 7,000 cots and 7,000 blankets, and is even reported to be standing by with dynamite and wrecking balls to destroy the bridge if the waters start to back up. The Amherst mayor predicted that a major flood could innundate 50 to 50 per cent of his town.

Sandbags and rowboats aside, federally subsidized flood insurance is practically the only protection home owners have against damage. Provisions for a flood insurance program were included in the 1968 Housing and Urban Development Act because insurance companiessed were refusing to writer such insurance on their own.

The program is supported approximately half and half by the government and 132 private companies grouped together as the National Flood Insurers Association.

Virtually all flood insurance is written today by members of this organization, the only exception being insurance written as part of large industrial contracts. In 1972, when Hurrican Agnes hit the East Coast, there were only 683 such policies in effect. In fiscal 1976 there were 793,779 policy holders.

Minimum coverage of $10,000 under the program costs $25 a year. The premium for maximum coverage of $35,000 on a home, plus $10,000 for the contents, is $123 annually.

In fiscal 1975 the government took in $41 million in premiums and paid out $26.3 million to about 11,000 claimants. The next year, which included Hurrican Eloise, saw almost twice as mnny claims filed and $74.6 million paid to beneficiaries. Premiums that year totaled only $58 million, for a net loss of $16.6 million.